The present invention relates to a catalyst for the production of an ethylene polymer using a trivalent alkyl chromium compound and a process for producing an ethylene polymer using the catalyst. More particularly, the invention relates to a catalyst for the production of an ethylene polymer using a trivalent alkyl chromium compound, a catalyst for the production of an ethylene polymer using a trivalent alkyl chromium compound and a tetravalent alkyl chromium compound and a process for producing an ethylene polymer using those catalysts.
It is well known that a catalyst for the production of an ethylene polymer obtained by supporting an organochromium compound on an inorganic oxide solid has inherent performances such that it is possible to adjust a molecular weight with hydrogen and an ethylene polymer obtained has a narrow molecular weight distribution, as being different from a so-called Phillips catalyst obtained by supporting a chromium compound on an inorganic oxide solid and then conducting activation in a non-reducing atmosphere such as air or oxygen.
Examples of such a catalyst include a so-called chromocene catalyst comprising bis(cyclopentadienyl)chromium(II) supported on silica or the like as disclosed in JP-A-50-68985, U.S. Pat. No. 3,879,368, U.S. Pat. No. 3,687,920, JP-B-50-68985, JP-B-52-31226, Canadian Patent 1,087,595, JP-B-58-25323, U.S. Pat. No. 4,101,445, U.S. Pat. No. 4,424,139 and Published Japanese Translation of International Patent Application Hei. 8-512339; a catalyst comprising dicumene chromium(0) supported on silica or the like as disclosed in JP-B-47-16647 and U.S. Pat. No. 4,364,841; a catalyst comprising bis(indenyl)chromium(II) or bis(fluorenyl)chromium(II) supported on silica or the like as disclosed in U.S. Pat. No. 4,015,059; a catalyst comprising (pentamethylcyclopentadienyl)(2-methylpentadienyl)chromium(II) supported on silica or the like as disclosed in U.S. Pat. No. 5,169,817; a catalyst comprising xcfx80-allyl chromium(II) supported on silica or the like as disclosed in JP-B-47-13002 and JP-B-47-26429; a catalyst comprising tetraneopentyl chromium(IV) supported on silica or the like as disclosed in U.S. Pat. No. 3,798,250; a catalyst comprising tetrakis(bicycloheptyl) chromium(IV) supported on silica or the like as disclosed in JP-A-47-16590; and a catalyst comprising octakis(alkylsilyl) tetrachromium(II) multinuclear complex supported on silica or the like as disclosed in U.S. Pat. No. 4,668,808. These catalysts use organochromium compounds in which the valency of the chromium atom is 0, 2 or 4, and there are only a few cases to use a trivalent organochromium compound.
Example of an ethylene polymer obtained by supporting a trivalent organochromium compound on an inorganic oxide solid is a catalyst comprising tris(bicycloheptyl)chromium(III) supported on silica as disclosed in, for example, JP-A-47-17753. In the case of this catalyst, tris(bicycloheptyl)chromium(III) which is a trivalent organochromium compound is unstable to heat and/or light and is liable to decompose. As a result, even if the compound is supported on silica to form a catalyst, the compound causes deterioration of activity with the passage of time and the catalyst cannot be stored. Thus it is difficult to obtain a catalyst having stable quality and an ethylene polymer. Further, a catalyst comprising a component comprising cyclopentadienyl chromium(III) hydrocarbyl compound supported on an inorganic carrier such as silica, and an alkyl aluminum compound is known as disclosed in U.S. Pat. No. 5,418,200, JP-A-7-502783, WO 96-27621 and WO 96-23006. In the case of this catalyst, deterioration with the passage of time is difficult to cause, but activity per catalyst and activity per chromium atom are very low. Thus, such a catalyst is poor economical property and is not suitable for industrial use. Further, since catalyst residue remains in the polymer obtained in a large amount, a product colors and deterioration is accelerated, thus giving adverse influence. Furthermore, since this catalyst is difficult to catalyze copolymerization with xcex1-olefin such as 1-butene and 1-hexene, it is difficult to control a density of an ethylene polymer. Thus, the catalyst has a great disadvantage that only a product having a very limited high density can be produced. As described above, there have been practical problems in the catalyst for the production of an ethylene polymer, obtained by supporting the trivalent organochromium compound on an inorganic oxide solid.
Therefore, a first object of the present invention is to provide a catalyst for the production of an ethylene polymer, which overcomes the problems of the catalyst using the trivalent organochromium compound, does not cause deterioration with the passage of time, is stable to heat and light, improves its activity, and is capable of catalyze copolymerization with xcex1-olefin, and a process for producing an ethylene polymer efficiently using the catalyst.
The ethylene polymer is generally widely used as a resin material for various molded articles, but the properties required in the ethylene polymer vary depending on the molding method and the purpose of use. For example, a product molded by an injection molding has a relatively low molecular weight, and use of a polymer having a narrow molecular weight distribution is suitable. On the other hand, a product molded by a blow molding or a film molding has a relatively high molecular weight, and use of a polymer having a broad molecular weight distribution is suitable. It has conventionally been known that an ethylene polymer having a broad molecular weight distribution suitable for a blow molding, a film molding and the like can be obtained using a Phillips catalyst prepared by supporting a chromium compound such as chromium trioxide, chromium acetate or tris(acetylacetonato)chromium on an inorganic oxide solid such as silica and activating the same in oxygen gas or air.
Further, an ethylene polymer having a broad molecular weight distribution suitable for a blow molding, a film molding and the like can also be obtained using a catalyst (silyl chromate catalyst) prepared by supporting bis (triphenylsilyl)chromate on an inorganic oxide solid such as silica and treating the same with an organoaluminum as disclosed in, for example, JP-B-44-2996, JP-B-44-3827 and JP-B-47-1766.
Thus, chromium catalysts have conventionally been used industrially as a catalyst for producing an ethylene polymer having high molecular weight, which is suitable for a blow molding or a film molding.
However, in the use of those catalysts, even if polymerization temperature is elevated as high as possible or a chain transfer agent such as hydrogen is used, in a process of a slurry polymerization in which the ethylene polymer does not dissolve in a polymerization solvent or a gas phase polymerization, it is still difficult to produce an ethylene polymer having low molecular weight suitable for an injection molding.
Improvement methods of Phillips catalyst for obtaining an ethylene polymer having low molecular weight have been proposed as disclosed in, for example, U.S. Pat. No. 4,248,735, U.S. Pat. No. 4,177,162, U.S. Pat. No. 4,151,122, U.S. Pat. No. 4,312,967, U.S. Pat. No. 4,397,765, U.S. Pat. No. 4,364,839 and U.S. Pat. No. 4,364,842. However, it cannot say that the molecular weigh can sufficiently be decreased by those methods. Thus, there has been a limit for the molecular weight of an ethylene polymer that can be produced by those catalysts.
On the other hand, according to the use of a chromocene catalyst comprising bis(cyclopentadienyl)chromium(II) supported on silica or the like as described before, an ethylene polymer having a very wide molecular weight range of from high molecular weight to low molecular weight can be obtained by using hydrogen as a chain transfer agent. However, use of the chromocene catalyst has a great disadvantage that since xcex1-olefin such as 1-butene or 1-hexene is not copolymerized, it is impossible to control a density of the ethylene polymer and only a product having very limited high density can be obtained. Therefore, if a chromium catalyst which can provide an ethylene polymer having low molecular weight and can control a density thereof in a wide range is obtained, the range of an ethylene polymer which can be produced by a chromium catalyst can be expanded, making it possible to produce products having industrial characteristics.
Further high quality is demanded for an ethylene polymer suitable for use in a blow molding or a film molding. In particular, in the case of producing a blow molded product using an ethylene polymer having a broad molecular weight distribution obtained using Phillips catalyst or silyl chromate catalyst, such a molded product is not always satisfied with either of the following points:
(1) Balance between rigidity and environmental stress crack resistance (ESCR) is not sufficient; and
(2) Since melt tension is not sufficient in molding, ununiformity occurs in the thickness of a blow molded product, and also the surface of a molded product roughens, which is undesirable on appearance.
In order to solve these problems, it is necessary to further broaden the molecular weight distribution.
As is known well, Ziegler catalyst enables the molecular weight to be easily controlled with hydrogen and can produce an ethylene polymer having a wide molecular weight range of from low molecular weight to high molecular weight, and use of Ziegler catalyst in a multi-stage polymerization using at least two reactors connected one after another enables the production of an ethylene polymer having a broad molecular weight distribution by, for example, producing a high molecular weight component in a first-stage reactor and then a low molecular weight component in a second-stage reactor as disclosed in, for example, JP-B-3-18645. However, the ethylene polymer by a multistage polymerization using Ziegler catalyst can improve balance between rigidity and ESCR, but melt tension of such a polymer is not comparable to that of an ethylene polymer obtained using Phillips catalyst or silyl chromate catalyst and thus moldability is poor. Therefore, if there is a chromium catalyst that can produce an ethylene polymer having a broad molecular weight range by a multistage polymerization, the problem on moldability can be solved. A process for producing an ethylene polymer by a two-stage polymerization using Phillips catalyst is disclosed in, for example, EP-A-905145 and EP-A-905146. However, such Phillips catalyst cannot produce a component having low molecular weight and there is a limit in broadening a molecular weight distribution as described above. Further, U.S. Pat. No. 5,478,898 discloses a process for conducting a two-stage polymerization using chromocene catalyst. This catalyst makes it possible to obtain an ethylene polymer having a very broad molecular weight distribution of from high molecular weight to low molecular weight by using hydrogen as a chain transfer agent and thus a molecular weight distribution can be broadened, as described before. However, since use of the chromocene catalyst does not enable xcex1-olefin such as 1-butene or 1-hexene to copolymerize, it is impossible to control a density of the ethylene polymer and only a product having high density is obtained. Thus it cannot be said that this is an industrially useful process. Therefore, if a chromium catalyst that can greatly broaden a molecular weight distribution by a multistage polymerization and can control a density in a wide range is obtained, it is possible to produce an industrially useful ethylene polymer suitable for used in blow molding or film molding.
A method of using a catalyst comprising two kinds of organochromium compounds supported on an inorganic oxide solid such as silica is known as a method of broadening a molecular weight distribution by using chromium catalyst. For example, U.S. Pat. No. 5,169,816 discloses a catalyst comprising bis-(cyclopentadienyl)chromium(II) and (pentamethylcyclopentadienyl)(2-methylpentadienyl)chromium(II) supported on silica. According to the use of this catalyst, a molecular weight distribution of an ethylene polymer obtained broadens as compared with the case of using each organochromium compound alone. However, a high molecular weight component formed by (pentamethylcyclopentadienyl)(2-methylpentadienyl)chromium(II) does not copolymerize with xcex1-olefin such as 1-butene or 1-hexene and as a result, balance between rigidity and ESCR is poor. Further, melt tension is not also sufficient.
U.S. Pat. No. 5,723,399 discloses a catalyst comprising a chromium compound such as chromium-carboxylic acid salt, chromium-1,3-diketo compound, chromic acid ester or chromium-amide compound, a carrier, aluminoxane and a transition metal compound having a group having a conjugated xcfx80-electron as a ligand. A molecular weight distribution of an ethylene polymer obtained using this catalyst broadens as compared with the case of using each transition metal component alone. However, xcex1-olefin such as 1-butene or 1-hexene is difficult to be copolymerized with a high molecular weight component formed by using the chromium compound and thus further improvement in balance between rigidity and ESCR is required.
JPA-A-10-338707 discloses a catalyst comprising two kinds of chromium compounds selected from chromium-carboxylic acid salt, chromium-1,3-diketo compound, chromic acid ester and chromium-amide compound, a carrier, aluminoxane and organic metal alkoxide and/or organic metal siloxide. A molecular weight distribution of an ethylene polymer obtained using this catalyst broadens as compared with the case of using each chromium compound alone. However, xcex1-olefin such as 1-butene or 1-hexene is difficult to be copolymerized with a high molecular weight component formed by using the chromium compound, and thus further improvement in balance between rigidity and ESCR is required.
Therefore, a second object of the present invention is to provide a novel catalyst for the production of an ethylene polymer, which overcomes the conventional problems in the chromium catalyst, and a process for efficiently producing an ethylene polymer having low molecular weight and narrow molecular weight distribution suitable for use in injection molding or an ethylene polymer having broad molecular weight distribution, excellent balance between rigidity and ESCR and excellent moldability suitable for use in blow molding or film molding, using the catalyst.
The inventors of the present invention have made intensive investigations in view of the above-described problems inherent in the prior art. As a result, it has been found that a catalyst comprising a specific trivalent alkyl chromium compound and an inorganic oxide solid, or a catalyst comprising a specific trivalent alkyl chromium compound, an inorganic oxide solid and an organoaluminum compound does not cause deterioration with the passage of time and can be used as a catalyst having improved activity for the production of an ethylene polymer which is copolymerizable with xcex1-olefin, thereby an ethylene polymer having excellent impact resistance is obtained.
It has also been found that if a specific tetravalent alkyl chromium compound is used together with the specific trivalent alkyl chromium compound and a catalyst comprising those compounds supported on an inorganic oxide solid is used, an ethylene polymer obtained has low molecular weight and also narrow molecular weight distribution as compared with the case of using a catalyst comprising a trivalent or tetravalent alkyl chromium compound and an inorganic oxide solid, under the same polymerization conditions. The present invention has been completed based on those findings.
The present invention provides the following catalyst for the production of an ethylene polymer and a process for producing an ethylene polymer:
1) A catalyst for the production of an ethylene polymer, comprising:
a trivalent alkyl chromium compound represented by the following formula (1): 
wherein R1, R2 and R3 which may be the same or different each represent hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group having 6 to 18 carbon atoms or a silyl group substituted with an alkyl group having 1 to 18 carbon atoms or an aryl group having 6 to 18 carbon atoms, provided that R1, R2 and R3 are not simultaneously hydrogen atom; and two of R1, R2 and R3 may be connected to form a ring, and
an inorganic oxide solid.
2) The catalyst for the production of an ethylene polymer as described in item 1) above, which is obtained by supporting the trivalent alkyl chromium compound represented by the formula (1) on the inorganic oxide solid.
3) The catalyst for the production of an ethylene polymer as described in item 2) above, which is obtained by supporting the trivalent alkyl chromium compound represented by the formula (1) in an amount of 0.01 to 10% in terms of chromium atom to the inorganic oxide solid, on the inorganic oxide solid.
4) The catalyst for the production of an ethylene polymer as described in item 1) above, wherein the trivalent alkyl chromium compound represented by the formula (1) is a trivalent alkyl chromium compound represented by the following formula (3): 
wherein R5 and R6 which may be the same or different each represent hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group having 6 to 18 carbon atoms or a silyl group substituted with an alkyl group having 1 to 18 carbon atoms or an aryl group having 6 to 18 carbon atoms, provided that R5 and R6 are not simultaneously hydrogen atom; and R5 and R6 may be connected to form a ring.
5) The catalyst for the production of an ethylene polymer as described in item 4) above, wherein the trivalent alkyl chromium compound represented by the formula (3) is a trivalent alkyl chromium compound represented by the following formula (4): 
wherein R7 and R8 which may be the same or different each represent an alkyl group having 1 to 18 carbon atoms or an aryl group having 6 to 18 carbon atoms; and R7 and R8 may be connected to form a ring.
6) A catalyst for the production of an ethylene polymer, comprising a trivalent alkyl chromium compound represented by the following formula (1): 
wherein R1, R2 and R3 are the same as defined above, an inorganic oxide solid, and an organoaluminum compound represented by the following formula (2):
(R4)nAl(X)3xe2x88x92nxe2x80x83xe2x80x83(2)
wherein a plurality of R4 which may be the same or different each represent an alkyl group having 1 to 18 carbon atoms, a plurality of X which may be the same or different each represent a halogen atom, an alkoxyl group, siloxy group or hydrogen atom and n is an integer of 1 to 3.
7) The catalyst for the production of an ethylene polymer as described in item 6) above, which is obtained by supporting the trivalent alkyl chromium compound represented by the formula (1) on the inorganic oxide solid and then reacting the solid catalyst component obtained with the organoaluminum compound represented by the formula (2).
8) The catalyst for the production of an ethylene polymer as described in item 7) above, which is obtained by supporting the trivalent alkyl chromium compound represented by the formula (1) in an amount of 0.01 to 10% in terms of chromium atom to the inorganic oxide solid, on the inorganic oxide solid and then reacting the solid catalyst component obtained with the organoaluminum compound represented by the formula (2) in the proportion such that a molar ratio of aluminum atom/chromium atom is 1 to 1000.
9) The catalyst for the production of an ethylene polymer as described in item 6) above, wherein the trivalent alkyl chromium compound represented by the formula (1) is a trivalent alkyl chromium compound represented by the following formula (3): 
wherein R5 and R6 are the same as defined in item 4) above.
10) The catalyst for the production of an ethylene polymer as described in item 9) above, wherein the trivalent alkyl chromium compound represented by the formula (3) is a trivalent alkyl chromium compound represented by the following formula (4): 
wherein R7 and R8 are the same as defined in item 5) above.
11) A catalyst for the production of an ethylene polymer, comprising a trivalent alkyl chromium compound represented by the following formula (1): 
wherein R1, R2 and R3 which may be the same or different each represent hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group having 6 to 18 carbon atoms or a silyl group substituted with an alkyl group having 1 to 18 carbon atoms or an aryl group having 6 to 18 carbon atoms, provided that R1, R2 and R3 are not simultaneously hydrogen atom; and at least two of R1, R2 and R3 may be connected to form a ring, a tetravalent alkyl chromium compound represented by the following formula (5) 
wherein R9, R10 and R11 which may be the same or different each represent hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group having 6 to 18 carbon atoms or a silyl group substituted with an alkyl group having 1 to 18 carbon atoms or an aryl group having 6 to 18 carbon atoms, provided that R9, R10 and R11 are not simultaneously hydrogen atom; and at least two of R9, R10 and R11 may be connected to form a ring, and an inorganic oxide solid.
12) The catalyst for the production of an ethylene polymer as described in item 11) above, wherein the trivalent alkyl chromium compound represented by the formula (1) and the tetravalent alkyl chromium compound represented by the formula (5) are supported on the inorganic oxide solid.
13) The catalyst for the production of an ethylene polymer as described in item 12) above, wherein the amount of the trivalent alkyl chromium compound represented by the formula (1) and the tetravalent alkyl chromium compound represented by the formula (5) supported on the inorganic oxide solid is 0.01 to 5% in terms of chromium atom to the inorganic oxide solid.
14) The catalyst for the production of an ethylene polymer as described in item 12) or 13) above, wherein the ratio of the trivalent alkyl chromium compound represented by the formula (1) and the tetravalent alkyl chromium compound represented by the formula (5) supported on the inorganic oxide solid is 1 to 99:99 to 1 in terms of mass ratio of chromium atom.
15) The catalyst for the production of an ethylene polymer as described in any one of items 11) to 14) above, wherein the inorganic oxide solid that has been calcined at 400 to 900xc2x0 C. is used.
16) A process for producing an ethylene polymer, which comprising using the catalyst for the production of an ethylene polymer as described in any one of items 1) to 5) above.
17) A process for producing an ethylene polymer, which comprising using the catalyst for the production of an ethylene polymer as described in any one of items 6) to 10) above.
18) A process for producing an ethylene polymer, which comprising using the catalyst for the production of an ethylene polymer as described in any one of items 11) to 15) above.
19) The process for producing an ethylene polymer as described in item 18) above, which comprises conducting a multi-stage polymerization by connecting at least two reactors.